1. Field of the Invention
The invention relates to the field of magnetron modulation and more particularly to modulators for use with magnetrons requiring negative pulses for activation.
2. Description of the Prior Art
A weather radar determines turbulence in a given region from the signal level and doppler shift of the signal returns. These parameters are representative of the reflectivity and velocity of the air mass in the illuminated region. The reflectivity, which is a measure of the air density, and the velocity are then processed to establish the turbulence in the region. Since a variation of frequency and/or amplitude from pulse-to-pulse can give rise to erroneous data, transmitters which provide signals with amplitudes and frequencies that do not vary from pulse-to-pulse are required. Additionally, frequency and amplitude stability are required for ground and sea clutter rejection.
Due to the requirement that sufficient warning be given of turbulence in a region, it is desirable to use a magnetron, with its high power output, as the transmitting device. The transmitted signal frequency and amplitude are functions of the current that flows through the magnetron during the transmission period. Consequently, to provide pulse-to-pulse frequency and amplitude stability the current must be tightly regulated. In the prior art pulse-to-pulse stability is generally provided by regulating the voltage amplitude of the pulse modulating the magnetron, thereby indirectly regulating the current. Since small variations in the applied voltage cause large variations in the current through the magnetron, the control of the pulse amplitude requires extremely tight voltage regulation which is difficult to achieve.
A system of the prior art which directly regulates the magnetron current is disclosed in U.S. Pat. 4,835,536, entitled "Weather Radar With Turbulence Detection" issued on May 30, 1989 to G. H. Piesinger and S. D. Neuharth and assigned to the assignee of the present invention. This system employes a positive pulse magnetron, that is the magnetron is fired by applying a positive pulse to its anode. The lifetime of such magnetrons is relatively short, being in the order of 500 hours of use. It is therefore desirable to replace the positive pulse magnetrons with the small negatively pulsed magnetrons having lifetimes in the order of 2000 hours which have recently become available. These negatively pulsed magnetrons are fired by applying a negative pulse to its cathode. A negative pulse magnetron, like the positive pulse magnetron, requires current stabilization to achieve pulse-to-pulse stability. Since the magnetron filament is coupled to the cathode in a negative pulse magnetron, another problem presented is the isolation of the d.c. filament power supply from the high negative voltage applied to the cathode.
Magnetrons may arc when fired. Though such arcing occurs only on a small percentage of the firings, when the arcing does occur damage to modulator switch which controls the pulsing of the magnetron results. Since the reliability of the system is a function of the reliability of all the system elements, it is desirable to provide circuitry that protects the switch.